man/io_uring_enter.2: improve timeout documentation
man/io_uring_enter.2: improve timeout documentation

- Missing mention of -ENOENT on timeout removal
- Mention the clock source used is CLOCK_MONOTONIC

Signed-off-by: Jens Axboe <cd8c6775e60d6f67a6984377324e5290df3d5358@kernel.dk>

axboe/liburing,axboe/liburing,axboe/liburing